Three-way catalytic converters which perform oxidation and reduction of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust of an automobile engine, convert these noxious substances most efficiently when an air-fuel ratio of the fuel mixture supplied to the engine coincides with a stoichiometric air-fuel ratio.
In an engine equipped with such a catalytic converter, the air-fuel ratio of the air-fuel mixture is controlled for example by an electronic control unit. The ignition timing is also controlled in close relation with the air-fuel ratio control. The ignition timing control is generally performed based on a map of basic ignition timing having engine load and engine rotation speed as parameters is pre-stored in a memory provided in the control unit.
A real engine load and engine rotation speed are detected, and when these signals are input to the control unit, the map is looked up so as to determine this basic ignition timing.
A spark plug in the engine connected to an ignition coil generates a spark in accordance with this basic ignition timing so as to ignite the air-fuel mixture in a combustion chamber of the engine when a primary current flowing in the ignition coil is interrupted.
The basic ignition timing is set in the vicinity of a minimum spark advance for best torque, or MBT, which is required to maximize the engine output torque with a view to improving fuel cost-performance.
To improve the precision of this ignition timing control, the map of basic ignition timing must first be set correctly for each engine which requires many matching experiments to be performed. In particular, the required basic ignition timing varies in lean burn systems where the engine is run at a leaner air-fuel ratio than the stoichiometric air-fuel ratio under predetermined conditions. In this case the required basic ignition timing varies in a lean burn region and a non-lean burn region. Further, in engines fitted with an exhaust gas recirculation (EGR) device, the required basic ignition timing also varies according to the exhaust gas recirculation state.
When a plurality of maps are used to satisfy these requirements, the number of matching experiments increases in direct proportion to the number of maps, and the memory storage capacity required to store these maps also increases.
In Tokkai Hei 2-245450 published by the Japanese Patent Office in 1990, a pressure sensor is provided facing an engine cylinder, and a map value of basic ignition timing is corrected so that a crank angle which maximizes the rate of increase of cylinder pressure coincides with a target value. In this way, it is possible to achieve high precision MBT control without performing a large number of matching experiments.
However in such a device, a pressure sensor must be provided in the combustion chamber and this increases the manufacturing cost of the device. The lifetime of the pressure sensor is also short in comparison to that of other engine parts.